The present invention relates to silicone molding compositions and more particularly the present invention relates to the mixing of silicone molding compositions utilizing a wet mixing process.
Silicone materials are well-known and are highly regarded for their high temperature capabilities, for their resistance to the elements and for their good low temperature properties. In addition, silicon compositions are good electrical insulating materials. In the fabrication of the electrical components, if often becomes necessary to incorporate electrical components such as diodes and resistors in a composition which can cure to a hard solid which composition can withstand high temperatures and allow the electrical elements to operate at high temperatures without the composition bursting into flame and which composition has the proper ingredients for electrical insulative properties. In addition, it is desirable that if the composition encasing the electrical component breaks or burns because of excessively high temperatures due to fire from whatever source that the encasing composition when it burns will not give off toxic gases.
An encapsulating composition for electrical components is desired for several reasons. One reason is to protect the electrical components from dirt and moisture, and another reason is to protect the electrical component from mechanical shock. Accordingly, the encapsulant material has to be water tight or waterproof and also be of a hard substance which is sufficiently tough so that the casing can take mechanical shock without breaking. In addition, the encapsulant for the electrical component has to be able to take cycling from high temperatures to relatively low temperatures, that is room temperature, or below, without breaking or crazing and it has to be able to withstand high temperatures without burning or supporting a combustion.
Accordingly, it is highly desirable to have an encapsulant material which meets the above requirements and also can allow a non-electrical component to operate at temperatures 350.degree. F. continuously without cracking or crazing and without supporting combustion.
Silicone molding compositions have been found especially suited to perform the above functions. Thus silicone molding compositions are especially suitable for encapsulating electrical components such as diodes and resistors and allowing such components to operate at temperatures of 350.degree. F. continuously without supporting combustion or cracking or crazing. Such silicone molding compositions also can cycle from low temperatures to high temperatures without cracking or crazing depending on the presence of certain silicone additives.
Examples of such silicone molding compositions are for instance to be found in the patent application of Duane F. Merrill Ser. No. 750,993 filed Dec. 15, 1976, entitled "Process for Producing Low Molecular Weight Silicone Resins" and the patent of Alekna U.S. Pat. No. 3,666,830, both of which are incorporated into the present case by reference.
Such silicone molding compositions generally comprise a base silicone resin composed of trifunctional siloxy units and difunctional siloxy units, various types of fillers which typically are some form of precipitated or fumed silica, a pigment, a catalyst system and a plasticizer. The plasticizer is typically a silicone fluid of low viscosity which acts as a plasticizing fluid in the composition and allows the composition to have toughness properties, that is that the composition when cured, will be hard but not excessively brittle. The plasticizing fluid also imparts the properties to the composition so that it can withstand cycling from extreme temperature limits without cracking or crazing. It can be understood that the silicone binder resin, the main constituent of the silicone composition can be any of a number of silicone binder resins, such as the two of the disclosures herein above given.
In addition, the catalyst system usually comprises some assortment of lead compounds by themselves or in combination with a carboxylic acid or a carboxylic acid anhydride. There may be added other ingredients to the compositions, such as pigments other stabilizing additives, etc. If it is desired to use the composition, the mixture of the above ingredients is taken, melted and injected molded around the electrical component in a matter of seconds in the appropriate injection molding machinery. To determine if the silicone molding composition will perform satisfactorily for the encapsulating of electrical components there are various tests such as gel tests and spiral flow tests that are carried out on the composition.
One of the difficulties in the formulation of such silicone molding compositions was the determination that the final silicone molding composition would have the properties that were desired or requested for a particular application. Accordingly, to meet such difficulties or problems in the formulation of silicone molding compositions various types of binder resins have been developed and have been applied in the formation of silicone molding compositions. Another problem which is important, and to which the present efforts are also directed is the preparation of a silicone molding composition, that is the mixing of the various ingredients to form silicone molding composition particles which are homogeneous.
It is desired to have a silicone molding composition which is a homogeneous mixture of the ingredients since all the particles will flow at the same rate in the injection molding machinery, and result in a uniform performance in the injection molding machine and in the reduction of rejects due to improper cure and melting of the silicone molding composition. Accordingly, it is highly desirable to mix all the ingredients in the silicone molding composition as uniformly as possible so as to arrive at a silicone molding composition particles of uniform quality. Accordingly, it has been the practice to take the ingredients and mix them in a Ferrel Mixing System in which the ingredients are press-mixed under pressure similar to a screw extruding system in which the ingredients are dry mixed. The product is then taken and ground up to form the desired particle size of the silicone molding composition.
There were several problems with such a mixing system. In silicone molding compositions that utilized glass fibers, such fibers were not uniformly mixed with the silicone resin in the dry system. In addition, the extrusion mixing system had to be frequently repaired or replaced because of the excessive wear of the mixer components due to the abrasive nature of the glass fibers and other ingredients in the silicone molding composition mix.
In addition, the mixing of the ingredients in the Ferrel Mixing System in a dry state resulted in extensively high temperatures being experienced by the mixed components while being processed through the mixture. That is at temperatures of 200.degree. C. or above. One disadvantage of such high temperatures was that there was a partial curing of the silicone molding composition, because of partial curing of the silicone binder resin and thus the silicone molding composition would not perform satisfactorily in the injection molding machine. The partial curing of the molding composition reduced the shelf-life of the molding composition, that is because of the partial curing, the molding composition would cure too quickly in the injection molding apparatus. Further, such partial curing of the molding composition would result in mixed compositions having different partial curing depending on the temperatures arrived at in the Ferrel Mixing System. As a result, mixed batches of different properties would result, and these would have to be combined to arrive at one batch with uniform properties. However, such properties would still vary from other batches that were mixed together and that were prepared subsequent or prior to the batches in question. Further, some compositions were partially cured to an extent that they were unworkable in injection molding machine, that is they would cure up as nodules in the silicone molding composition as soon as it was melted in the injection molding machine and such nodules could block up the ports in the molding equipment and result in malfunction.
Accordingly, for these reasons many batches did not meet the quality control requirements, that is there was too much partially cured molding composition in the batch, and the batch would then be discarded. Accordingly, it is one object of the present invention to have a simple and economic system for the mixing of the ingredients in the silicone molding composition.
It is an additional object of the present invention to have an economic and simple system for the mixture of ingredients in the silicone molding composition where there is no partial cure of the molding composition during the mixing.
It is still an additional object of the present invention to have a process for mixing the ingredients in a silicone molding composition by utilizing a solvent such that there is not excessive wear in the mixing apparatus and such that the mixing can be carried out without much difficulty.
It is still an additional object of the present invention to provide for the wet-mixing of the ingredients in the silicone molding composition utilizing a solvent such that the mixture of ingredients is carried out without giving off undue excesses of solvent to the atmosphere and which mixing is carried out in a continuous manner. These and other objects of the present invention are accomplished by means of the disclosure set forth herein below.